The film star packed on a total of 38 pounds (17 kilograms) while making Roadie - and he was amazed how easy it was to get fat.
He tells WENN, "I would eat late at night; every night I would have many slices of pizza, milkshakes, candy, french fries and fried food.
"I ate well during the day so I wouldn't fall asleep at work. I was so uncomfortable. I was sweaty but it was great for the role. Every day I would go to craft service and load up bowls of candy and cookies."
Eldard is now losing the weight, so he can portray a washed-up rocker in a new film based on his own screenplay.
He adds, "I'm actually writing a script about a rock star that I hope to star in and direct. It's about a guy who was a star and the business has changed so and he has a major substance abuse problem. It's a road trip movie. It's his struggle to come to terms with what is going on in his life."